Densitometer

ABSTRACT

A vibration densitometer including an electromechanical oscillator having an automatic phase control.

BACKGROUND OF THE INVENTION

This invention relates to vibration densitometers, and more particularlyto a less complicated, more accurate densitometer of a 40 percent largerrange.

Vibration densitometers have a complicated construction, a limitedaccuracy, and a limited range.

SUMMARY OF THE INVENTION

In accordance with the present invention, the above-described and otherdisadvantages of the prior art are overcome by providing an automaticphase control.

The above-described and other advantages of the present invention willbe better understood from the following detailed description whenconsidered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are to be regarded as merely illustrative:

FIG. 1 is a block diagram of a densitometer constructed in accordancewith the present invention;

FIG. 2 is a block digram of a loop circuit shown in FIG. 1;

FIG. 3 is a schematic diagram of an input circuit, an AGC amplifier, atracking filter, two zero crossing detectors, two phase detectors, twolow pass filters and a clamp shown in FIG. 2; and

FIG. 4 is a schematic diagram of a phase detector shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, in FIG. 1, a vibration densitometer probe is indicatedat 34' having a driver coil 23, a vane 24, a piezoelectric crystal 25and a preamplifier 26.

Probe 34' has an input lead 27 and an output lead 28.

Other blocks shown in FIG. 1 are a loop circuit 29, a digital functiongenerator 30 and utilization means 31. Loop circuit 29 has an input lead32 and output leads 33 and 34. Digital function generator 30 has aninput lead 35 connected from loop circuit output lead 34. The output ofdigital function generator 30 is connected to utilization means 31.

The output lead 28 of probe 34' is connected to the input lead 32 ofloop circuit 29. The input lead 27 of probe 34' is connected from theoutput lead 33 of loop circuit 29. Probe 34' and loop circuit 29 form aclosed loop electromechanical oscillator. Vane 24 is submerged in afluid. The density of the fluid is a function of the frequency at whichvane 24 vibrates.

Digital function generator 30 may have its input lead 35 connected fromlead 33 or at other points in loop circuit 29. Loop circuit 29 impressesa square wave voltage on input lead 35 of digital function generator 30having a mark-to-space ratio of 1:1.

Utilization means 31 shown in FIG. 1 may be a density indicator, aspecific gravity indicator, a process controller or otherwise.

Probe 34' digital function generator 30 and utilization means 31 may beidentical to those disclosed in U.S. Pat. No. 3,878,374. The same istrue of everything shown in FIGS. 2 and 3, but not 4 except phasedetector 50' shown in FIG. 2 and the connection therefrom shown in FIG.3.

Probe 34' shown in FIG. 1 may be conventional.

Preamplifier 26 shown in FIG. 1 may be conventional. Preamplifier 26 mayalso be conventional.

Loop circuit 29 is shown in FIG. 2 including an input circuit 36, an AGCamplifier 37, a tracking filter 38, a zero crossing detector 39, aone-shot multivibrator 40, an inverter 41, a clamp 42, a phase lock loop43, a squarer 44, an AND gate 45, an inverter 46, a phase lock loop 47and a driver amplifier 48 connected in succession as serial stages frominput lead 32 of input circuit 29 to its output lead 33 and connectedrespectively from the output lead 28 of probe 34' and to the input lead27 of probe 34'.

In FIG. 2, other stages are a zero crossing detector 49, a phasedetector 50, a low pass filter 51, a phase detector 52, a low passfilter 53, a threshold detector 54, an inverter 55, a clamp 56, a sweeposcillator 57, an emitter-follower 58, a saw-tooth generator 59 and aphase adjustment circuit 60 (manually adjustable only).

AGC amplifier 37 has an AGC input lead 61 connected from the output ofclamp 56.

Tracking filter 38 has two output leads 62 and 63. Tracking filteroutput lead 63 is connected to the input of zero crossing detector 49.The output of zero crossing detector 49 is connected to one input 64 ofphase detector 50. A junction is provided at 65 from which an outputlead 66 of AGC amplifier 37 is connected. Tracking filter 38 has twoinput leads 67 and 68. Tracking filter input lead 67 is connected fromjunction 65. Lead 68 is connected from the output of low pass filter 51.

Phase detector 50 has a second input lead 69 connected from junction 65.The output of phase detector 50 is connected to one input of low passfilter 51. Phase detector 50' has one input connected from the output ofinput circuit 36 and another input connected from lead 33. Phasedetector 50' has an output connected to another input of low pass filter51.

The purpose of zero crossing detector 49, phase detector 50 and low passfilter 51 is to cause tracking filter 38 to track the frequency ofoutput signal of AGC amplifier 37. Phase detector 50' causes the outputof driver amplifier 48 to track the phase (have a constant phasedifference relative to that) of the output signal of input circuit 36.The signal on the tracking filter 68, thus, causes the passband thereofto straddle the frequency of the input to tracking filter 38 over inputlead 67, and to track the phase thereof as well.

The output of tracking filter 38 on output lead 62 thereof is 90 degreesout of phase with the signal on the output lead 63 thereof. The signalfrom the tracking filter output lead 62 is impressed upon zero crossingdetector 39 and phase detector 52. The output of zero crossing detector39 is impressed both upon phase detector 52 and one-shot 40. The outputof phase detector 52 is impressed upon low pass filter 53.

A junction is provided at 70 connected from the output of low passfilter 53. A lead 71 is connected from junction 70 to input circuit 36to the AGC input of an amplifier therein for automatic gain control.

Threshold detector 54 has an input connected from junction 70. Thisinput of threshold detector 54, when below a predetermined potential,causes the potential of the ouput lead 73 of threshold detector 54 to goeither high or low. The output lead 73 of threshold detector 54 is,thus, for example, either ground or +15 volts or +V1. When the output oflow pass filter 53 is below the predetermined potential, output lead 73of threshold detector 54 is at ground.

Threshold detector 54 operates both of the clamps 42 and 56 and thesweep oscillator 57. Clamp 56 and sweep oscillator 57 are operatedthrough the inverter 55.

Inverter 55 has an output lead 74 which also assumes potentials of V1 orground.

Clamp 42 either passes the output of inverter 41 to the phase lock loop43 or in the other state of the threshold detector 54, clamp 42 havingan output lead 75, is operated to clamp the output lead 75 to ground.The output of inverter 55 is simply the inverse of the output ofthreshold detector 54. When the output of inverter 55 is high, sweeposcillator 57 receives power. When the output of inverter 55 is low, theoutput sweep oscillator 57 is at ground.

Emitter follower 58 is connected between sweep oscillator 57 and phaselock loop 43. Phase lock loop 43 has an output lead 76 which isconnected to squarer 44. Junctions are provided at 77 and 78. Squarer 44has an output lead 79 connected to junction 78. Junction 78 is connectedto junction 77. Clamp 56 is connected from junction 77 to AGC amplifierinput lead 61.

When the output of threshold detector 54 is high, loop circuit 29 istracking and opens clamp 42 to unground the output lead 75 thereof.Conversely, at the same time, inverter 55 grounds the input to sweeposcillator 57 and disables it. During tracking, inverter 55 alsodisables the output of clamp 56 by a connection 80 from inverter outputlead 74 to clamp 56.

During searching, threshold detector 54 holds the output of clamp 42 atground while inverter 55 operates sweep oscillator 57 and clamp 56passes the output of squarer 44 to the AGC input lead 61 of AGCamplifier 37.

In FIG. 2, junction 77 is connected to digital function generator 30shown in FIG. 1.

AND gate 45 receives an input from junction 78 and from an output lead81 of phase adjustment circuit 60.

Sawtooth generator 59 has an input lead 82 connected from junction 78,and an output lead 83 connected to an input of phase adjustment circuit60.

Circuit 60 is manually adjustable to manually adjust the sine wavecomponent of the output voltage of driver amplifier 48 through the useof certain structures including the phase adjustment circuit 60, itself,and phase lock loop 47. This adjustment can make the electromechanicaloscillator oscillate with maximum efficiency.

OPERATION

In the embodiment of the invention shown in FIG. 1, probe 34' and loopcircuit 29 provide an electromechanical oscillator which oscillates at afrequency dependent upon the density of the fluid in which vane 24 isimmersed. The same is true of the pulse repetition frequency of thesquare wave voltage applied to the input lead 35 of digital functiongenerator 30.

Digital function generator 30 may be described as a digitallinearization circuit. It produces a digital output directlyproportional to density from the input signal thereto impressed upon theinput lead 35 thereto.

In FIG. 3, input circuit 36 is shown for connection from preamplifier 26in FIG. 1. Input circuit 36 has input leads 84 and 85. Input circuit 36has various junctions 86, 87, 88, 89 and 90. A capacitor 91 is connectedfrom input lead 84 to junction 86. Input lead 85 is connected tojunction 87. A resistor 92 is connected between junctions 86 and 87. Atransformer 93 is provided with a primary winding 94 and a secondarywinding 95. Primary winding 94 is connected between junctions 86 and 87.Secondary winding 95 has leads 96 and 97, lead 97 being grounded. Apotentiometer 98 is provided having a winding 99 and a wiper 100.Winding 99 is connected from transformer secondary lead 96 to ground.Wiper 100 is connected to junction 88. A diode 101 is connected fromjunction 88 to ground and poled to be conductive in a direction towardground. A diode 102 is connected from junction 89 to ground and poled ina direction to be conductive toward junction 89. Junctions 88 and 89 areconnected together.

A capacitor 103 is connected from junction 89 to the non-inverting inputof a differential amplifier 104. Junction 90 is connected from theinverting input of amplifier 104. A capacitor 105 is connected fromjunction 90 to ground. A resistor 106 is connected from junction 70 tojunction 90 (FIG. 2).

All of the blocks shown in FIG. 2 may be entirely conventional.

In FIG. 3, a calibration frequency may be provided over input lead 107,if desired, and impressed upon junction 65 through a circuit 108.Circuit 108 includes junctions 109 and 110. A resistor 111 and acapacitor 112 are connected in series in that order from lead 107 tojunction 109. A diode 113 is connected from ground to junction 109, andis poled to be conductive in a direction toward junction 109. A diode114 is connected from junction 110 to ground and is poled to beconductive in a direction toward ground. Junctions 109 and 110 areconnected together. Junctions 110 and 65 are also connected together.

AGC amplifier 37 has junctions 115, 116, 117 and 118. A capacitor 119 isconnected from an output lead 120 of amplifier 104 in input circuit 36to junction 115. A resistor 121 is connected from junction 115 toground. Junction 115 is also connected to the non-inverting input of anamplifier 122.

Clamp 56 includes diodes 123 and 124, and a resistor 125. Diodes 123 and124 are connected in succession in that order from lead 80 to lead 61.

A junction is shown at 126. The anodes of diodes 123 and 124 areconnected to junction 126. The cathode of diode 123 is connected to lead80. The cathode of diode 124 is connected to lead 61.

Junction 77 is connected from junction 78, as described previously.

In FIG. 3, in AGC aplifier 37, a resistor 129 is connected betweenjunctions 117 and 118. Junctions 116 and 118 are connected together. Aresistor 128 is connected from junction 118 to ground. A resistor 127 isconnected from lead 61 to junction 116. Amplifier 122 has an output lead130 connected to junction 117. A capacitor 131 and a resistor 132 areconnected in series in that order from junction 117 to junction 65.

Again, in FIG. 3, junctions are provided at 133, 134, 135, 136 and 137.Junction 133 is connected from lead 67. A resistor 138 is connected fromjunction 133 to ground. A resistor 139 is connected between junctions133 and 134. A capacitor 140 is connected betweeen junctions 134 and136. A resistor 141 is connected between junctions 135 and 136. Adifferential amplifier 142 is provided having an inverting inputconnected from junction 136, a grounded non-inverting input, and anoutput lead 143 connected to junction 135.

Tracking filter 38 is connected to zero crossing detector 39 and phasedetector 52 via a lead 144 connected from junction 135 in trackingfilter 38 to a junction 145. Zero crossing detector 39 and phasedetector 52 are connected from junction 145. In FIG. 3, tracking filter38 has a field effect transistor 146 including a source 147, a drain 148and a gate 149. Source 147 is grounded. A resistor 150 is connected fromdrain 148 to junction 137. Junctions 134 and 137 are connected together.A resistor 151 is connected from junction 137 to ground. A resistor 152is connected from gate 149 to lead 68.

Zero crossing detector 49 has junctions at 153 and 154. A capacitor 155is connected from junction 137 to junction 153. A third junction 156 isalso provided and maintained at potential +V2. A resistor 157 isconnected between junctions 153 and 156. A resistor 158 is connectedfrom junction 156 to the non-inverting input lead of a differentialamplifier 159. Junction 153 is connected to the inverting input lead ofamplifier 159. Amplifier 159 has an output lead 160 connected tojunction 154. A resistor 161 is connected from junction 154 to potential+V2.

Lead 64 connects junction 154 to the input of a conventional amplifier162 in phase detector 50. Phase detector 50 also includes a conventionalelectronic or transistor switch 163 which is connected from and operatedby amplifier 162. Switch 163 is connected by a lead 69 from junction 65to low pass filter 51 at junction 164 therein. Low pass filter 51 hasvarious other junctions 165, 166, 166', 167 and 168. A resistor 169 isconnected from junction 164 to ground. A resistor 170 is connectedbetween junctions 164 and 165. A capacitor 171 is connected fromjunction 165 to ground. A resistor 172 is connected between junctions165 and 167. Junctions 166 and 167 are connected together. Apotentiometer is provided at 173 having a winding 174 and a wiper 175.Winding 174 is connected between +V1 and -V1. A resistor 176 isconnected from wiper 175 to junction 166'. Junctions 166 and 166' areconnected together. Junction 166' is also connected from phase detector50'. A differential amplifier 177 is provided having an output lead 178connected to junction 168. A capacitor 179 is connected betweenjunctions 166 and 168. Junction 167 is connected to the inverting inputlead of amplifier 177. The non-inverting input lead of amplifier 177 isconnected to ground. Lead 68 and resistor 152 are connected in series inthat order from junction 168 to gate 149 of field effect transistor 146.

Zero crossing detector 39 includes four junctions 180, 181, 182 and 183.A capacitor 184 is connected from junction 145 to junction 180. Aresistor 185 is connected between junctions 180 and 181. An amplifier isprovided at 186. A resistor 187 is connected from junction 181 to thenon-inverting input of amplifier 186. Junction 180 is connected to theinverting input of amplifier 186. Amplifier 186 has an output lead 188connected to junction 183. Junctions 181 and 182 are connected together.A resistor 189 is connected from junction 182 to potential +V1. Aresistor 190 is connected from junction 183 to potential +V2. A zenerdiode 191 is connected from junction 182 to ground and is poled to beback biased between potential +V1.

Phase detector 52 may be identical to phase detector 50 and, therefore,will not be described except that phase detector 52 has an input lead192 connected from junction 145 to a switch 193 via a resistor 194.Switch 193 is connected to low pass filter 53 to a junction 195 via adiode 196 poled to be conductive toward junction 195. Low pass filter 53also has junctions 197, 198 and 199. A resistor 200 is connected fromjunction 195 to ground. A capacitor 201 is connected from junction 197to ground. Junctions 195 and 197 are connected together. A differentialamplifier 202 has an output lead 203 connected to junction 198. Junction197 is connected to the non-inverting input of amplifier 202. Theinverting input of amplifier 202 is connected from junction 199.Junctions 198 and 70 are connected together. A resistor 204 is connectedbetween junctions 198 and 199. A resistor 205 is connected from junction199 to ground .

Although phase detector 50' shown in FIG. 2 may be any conventionalphase detector, it may be also of the type illustrated in FIG. 4 wherejunctions are provided at 300, 301, 302, 303, 304, 305, 306, 307, 308,309, 310, 311 and 312.

A resistor 313 is connected from input circuit 36 to junction 300.Differential amplifiers are provided at 314, 315, 316 and 317. Theinverting input of amplifier 314 is connected from junction 300. Thenon-inverting inputs of all of the amplifiers 314, 315, 316 and 317 areconnected to ground. A resistor 318 is connected between junctions 300and 301. A capacitor 319 is connected between junctions 301 and 302.Amplifiers 314, 315, 316 and 317 have respective output leads 320, 321,322 and 323 connected respectively to junctions 301, 304, 310 and 312. Aresistor 324 is connected from junction 303 to ground. The invertinginput leads of amplifiers 315, 316 and 317 are connected respectivelyfrom junctions 303, 309 and 311. A resistor 325 is connected fromjunction 304 to a positive potential. A resistor 326 is connected fromjunction 305 to ground. A resistor 327 is connected between junctions304 and 305. A field effect transistor (FET) is provided at 328. FET 328is provided with a source 329 and a drain 330 connected from junctions306 and 305, respectively. A capacitor 331 is connected from junction306 to ground. A resistor 332 is connected between junctions 306 and307. A capacitor 333 is connected between junction 307 and ground. Aresistor 334 is connected between junctions 307 and 309. Junctions 308and 309 are connected together. A potentiometer is provided at 335including a winding 336, and a wiper 337. A resistor 338 is connectedfrom wiper 337 to junction 308. A single-pole, double-throw switch isprovided at 339 including a pole 340, a contact 341 and another contact342. Contact 341 is connected from junction 309. A resistor 343 isconnected from contact 342 to low pass filter 51. POle 340 is connectedfrom junction 310. A capacitor 344 is connected between junctions 308and 310.

FET 328 also has a gate 345 connected from junction 312 via a resistor346. Phase detector 50' shown in FIG. 4 has an additional input lead347. A capacitor 348 and a resistor 349 are connected in series in thatorder from input lead 347 to junction 311. A resistor 350 is providedand is connected from junction 311 to ground. A resistor 351 isconnected from junction 312 to a positive potential.

Another potentiometer 352 is provided including a winding 353 and awiper 354. A resistor 355 is connected from wiper 354 to junction 302.

The position of wiper 354 can be adjusted so as to adjust the averagevalue of the waveform which appears at junction 302. The position ofwiper 337 is adjusted to adjust the fixed phase shift, if any, betweenthe signals received by phase detector 50' in FIG. 4 from input circuit36 and driver amplifier 48. Circuit 36 and amplifier 48 are shown inFIG. 2.

In accordance with the foregoing, the use of phase detector 50' makes itpossible to increase the accurate density indication of the densitometerof the present invention over a much larger span. For example, it iscommon for a densitometer of the prior art to operate accurately over aspan of from 0 to 5 pounds per cubic foot. On the other hand, thedensitometer of the present invention as disclosed herein provides anaccurate density indication over a span of from 0 to 7 pounds per cubicfoot, an increase in range of 40 percent.

In addition to the foregoing, it is sometimes necessary to computedensity from the formula

    d = AT.sup.2 + BT + C                                      (1)

it is much easier, less complicated and less expensive to computedensity by the formula

    d = AT.sup.2 + C                                           (2)

the present invention allows computation in accordance with equation(2).

In the foregoing:

d is density, T is the vane period and A, B and C are constants.

For example, see U.S. Pat. No. 3,769,831.

What is claimed is:
 1. A densitometer comprising: a probe having aninput lead and an output lead; a loop circuit having an input leadconnected from said probe output lead, a first output lead connected tosaid probe input lead, and a second output lead, said probe and saidloop circuit forming an electromechanical oscillator, said probeincluding a vane, means connected from said probe input lead to vibratesaid vane, a transducer, means connecting said transducer to said probeoutput lead to produce a first A.C. signal thereon of the same frequencyas that at which said vane vibrates, said first A.C. signal having afrequency which is a known invariant function of the density of thefluid in which said vane is immersed, a second A.C. signal beingimpressed upon said loop circuit second output lead of the samefrequency as that of said first A.C. signal; utilization means; outputmeans connecting said loop circuit second output lead to saidutilization means; a phase detector for adjusting the phase of saidsecond signal so that it remains the same relative to said first signalindependent of changes in said probe, said loop circuit or said fluid,said phase detector having first and second input leads and an outputlead, said loop circuit including an input circuit having an input leadconnected from said probe output lead, and an output lead connected tosaid phase detector first input lead, a tracking filter having first andsecond input leads and an output lead, means connecting said trackingfilter output lead to said loop circuit first output lead, said loopcircuit second output lead being connected to said phase detector secondinput lead, a low pass filter having an input lead and an output lead,means connecting said input circuit output lead to said first input leadof said tracking filter, said low pass filter output lead beingconnected to said second tracking filter input lead, said phase detectoroutput lead being connected to said low pass filter input lead.
 2. Theinvention as defined in claim 1, wherein said output means includes afunction generator.